It is well-known in the art to form electrode substrates of a foraminous or latticed electrically conductive material which serves as a support for the active material of the cell. These electrode substrates may be extremely thin (of the order of 0.08 mm.), generally planar in form and adapted to conform to the general cross section of the battery in which they are stacked. For example, it is known to provide nickel hydrogen batteries with circularly shaped hydrogen-reacting electrodes in a cylindrically shaped housing, wherein the substrates are formed by chemically etching nickel foil in a known manner to form a lattice, to which a platinum slurry is then adhered. The electrodes are arranged as a stack with like electrodes interconnected via conducting tabs provided at the periphery of each substrate. Each tab further serves as a means of support for its electrode in the cell stack. A hole is provided through the center of each electrode to align the electrodes in the stack.
As shown in U.S. Pat. No. 4,250,235, it is also known to make such circular electrodes out of etched nickel sheet material, and providing the substrate with the conducting and support tab located at a point adjacent the center of the substrate.
In general, it has been the practice to shape the substrate lattice of such prior art electrodes in the form of a conductive open mesh screen having a series of concentric circular, regularly spaced portions centered on a central alignment hole, and which are intersected or connected by other portions extending radially from the alignment hole. These intersecting radial and circular portions define segment areas between them which decrease in area toward the center of the substrate. Superimposed upon this primary pattern is a narrower secondary pattern of thinner radial and circular conducting portions dividing the sections formed by the primary pattern into a mesh of still smaller segmented areas. A tab by which the electrode is electrically connected to other electrodes is provided either at the perimeter of the lattice or at the central alignment hole.
On the one hand, in conducting electric current collected to a battery terminal, electrical resistance loss in conduction reduces the cell efficiency by sapping cell voltage while also adding heat caused by the metal's resistance. On the other hand, it is necessary to maximize the open areas of the electrode substrate within which the active material is carried in order to maintain adequate ion transfer in the cell stack and optimize the collection capacity of the electron collector. Hence, the expanded lattice form for the electrode substrate.
In the above-described prior art substrate lattice wherein the tab is on the perimeter, the current flow within the lattice is not directed or "focused" toward the conductive tab. Rather, the geometric focus of the lattice is the central alignment hole, which is not electrically active. The current carrying capacity of the electrode is thus not used effectively in that the current carrying area increases in the direction away from the tab, rather than in the direction toward it.
So, too, with the tab located adjacent the center of the substrate, as in U.S. Pat. No. 4,250,235, only a small proportion of the electron flow along the radial portions follows a straight route to the tab; rather, the radial portions terminate in a relatively narrow inner ring defining the center aperture, which then provides a part of the electron flow area to the tab.
Another deficiency presented by these prior art lattices is that there is little structural strength provided in the area of the tab, which is the point of attachment of the electrode in the cell stack, and consequently a point of stress on the electrode. It has been observed that the tabs are sometimes torn off the electrodes, thus eliminating the usefulness of those electrodes within the cell stack.
Yet another difficulty with these prior art forms of substrate lattices is that they provide non-uniform areas for carrying the active material. The areas defined by the primary lattice "radials" and "circumferentials" diminish toward the center of the lattice; the ratio of the metal area to the open area sharply increases. This inefficiently uses the electron collection capacity of the electrode due to the non-uniformity in adhered active material-substrate contact areas, and gives rise to undesirable non-uniform current densities across the electrode. The smaller lattice areas also present difficulties in the proper adhesion of the catalyst or active material, particularly adjacent the center aperture of the electrode.